This is an old idea, and one that died when Polaroid film came out. A single bath of chemicals to develop a film as quickly and simply as possible (and used on direct positive photographic paper you have the necessary means for those once ubiquitous photobooths that used to take our passport photos in a simpler age). The trick is to balance the rate of development with that of fixing. B&W developer is doing one job: depositing growing metallic silver crystals on the nidus of a silver bromide molecule that has been reduced to silver by a photon. Fixer comes along afterwards, usually, and dissolves the unexposed silver bromide from the film's emulsion. In the end, you have metallic silver where there was light, and clear film where there was darkness. This is a negative, and exposing photo paper through it will make a negative of that, which by some alchemical arithmetic is a positive. Now for a monobath, we need the developer to work on the exposed areas, while the fixer at the same time is is working on the unexposed. Remember, that unless you photograph things that are literally just black and white, no greys, all areas of the film have some exposure and it's just the amount that varies. Most fixers these days are ammonium thiosulphate and are buffered to be acidic, which makes them work a bit faster. If we make our monobath strongly alkaline, we can slow the fixer and let the developer have a crack at the exposed silver halides before the fixer dissolves them. Donald Quall is a gent who has been experimenting with making such a brew, since the last commercial monobath hasn't been sold for years. He uses lots of ammonia to get a pH of 9.5 (wear gloves!) Take 16ml of Kodak HC-110 US Concentrate, and add 180 ml water, then 10ml Ilford Rapid Fixer concentrate, then 50ml household ammonia (usually 5% solution ammonium HCl) and mix well. Warm to 24-26ºC, then develop negatives in total darkness for six minutes with no agitation. It is said to be OK to turn the light on after two minutes, but I didn't. Try to hold your breath for the six minutes as the ammonia fumes will be strong. Then was for five minutes with room temp water. The result is very fine-grained and quite contrasty.

Next weird experiment is in train. I have cut some 10x8 photographic paper into quarters in the dark (safelight OK as orthochromatic) and loaded it into film holders for the 4x5. ISO will be 3. Yes, just 3 - some modern digital cameras can make an image at ISO 25,600. These can be used as paper negatives for contact prints, scanned and then inverted to get the image desired, or even developed by a reversal process that will make a positive image rather than a negative (think slide film versus negative film). Might be fun.

Yes, and since modern films are panchromatic you cannot use a safelight. You probably know that photographic paper is orthochromatic (not very sensitive to red light) and so a safelight can be used. That's deliberately done purely for the convenience of being able to use a safelight - after all, you are only photographing black, white and various greys with a piece of photo paper under an enlarger with a B&W negative in it. Taking photos of the real world in all its colours is better with a film stock sensitive to all of them. Kodak stopped making orthochromatic film around 1930. Trivia note: orthochromatic movie film posed special problems. Red lips were black, blue eyes were white etc. Apparently actors were made up in un-lifelike ways to counter this effect. Unfortunately we don't have colour photographs of them in their weird make-up for fairly obvious reasons!

There's nothing to stop you using this solution in a tank, but it would mean using a greater volume. My tanks need about 500ml for a 120 film and 900ml to cover 4x5 negs. I could have mixed up four times as much, but 48ml of HC-110 syrup and 40ml of fixer concentrate? That gets expensive! I did this for fun, and don't see it as a great boon. After all, people who use film and develop it themselves in the digital age are not primarily interested in convenience!

That makes sense. When you said no agitation, my first thought was that the tank would have to be full enough to cover the film reel. Then when you said complete darkness, I figured you had to be developing your film in a tray.

I heard of tray developing when I was a kid, and my impression was that one ran the roll of film, back and forth, through the developer in the tray. (I never was sure about that or why anyone would do it that way for roll film.) Later I heard about development by inspection, and I really can't quite conceive of how that is done.

Not a roll, but sheets of film. Development by inspection has to be done under a safelight and is thus for orthochromatic photo paper. You can do the monobath in a tank with a reel, but you would need to make a larger volume.

I've been playing with Donald Qualls' monobath in 4x5 as above, and now tried it for the first time on a 35mm film, using some Plus-X. All I read says don't agitate it, but almost every frame has drag marks by the sprocket holes. I'll do it again tomorrow and agitate it.

Nikon F6, 50mm/f1.4, Plus-X, monobath, KM5400 II:

For the adventurous, the recipe to make 300ml for a 35mm tank is
19ml HC-110 US concentrate
12ml Ilford Rapid Fixer
59ml household ammonia
210ml water
6 minutes at 27ºC/80ºF, then wash and dry.